Method and apparatus for managing volatile organic content in polyolefin

ABSTRACT

The present invention relates generally to polyolefin production and to reducing volatile organic content (VOC) associated with the polyolefin. Techniques include the construction and implementation of a purge column model to calculate or estimate the VOC content in the polyolefin exiting the purge column. The techniques facilitate the design and operation of the polyolefin manufacturing process.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/093,588 filed on Apr. 25, 2011, which is a divisional ofU.S. patent application Ser. No. 11/895,712 filed on Aug. 27, 2007, nowU.S. Pat. No. 7,957,947 issued on Jun. 7, 2011, which claims the benefitunder 35 U.S.C. §119(e) of U.S. Provisional Patent Application No.60/840,163 filed on Aug. 25, 2006, each of which are incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to polyolefin production and toreducing volatile organic content (VOC) associated with the polyolefin.Techniques include the construction and implementation of a purge columnmodel to calculate or estimate the VOC content in the polyolefin streamexiting the purge column. The techniques facilitate the design andoperation of the polyolefin manufacturing process.

2. Description of the Related Art

This section is intended to introduce the reader to aspects of art thatmay be related to aspects of the present invention, which are describedand/or claimed below. This discussion is believed to be helpful inproviding the reader with background information to facilitate a betterunderstanding of the various aspects of the present invention.Accordingly, it should be understood that these statements are to beread in this light, and not as admissions of prior art.

As chemical and petrochemical technologies have advanced, the productsof these technologies have become increasingly prevalent in society. Inparticular, as techniques for bonding simple molecular building blocksinto longer chains (or polymers) have advanced, the polymer products,typically in the form of various plastics, have been increasinglyincorporated into various everyday items. For example, polyolefinpolymers, such as polyethylene, polypropylene, and their copolymers, areused for retail and pharmaceutical packaging, food and beveragepackaging (such as juice and soda bottles), household containers (suchas pails and boxes), household items (such as appliances, furniture,carpeting, and toys), automobile components, pipes, conduits, andvarious industrial products.

Specific types of polyolefins, such as high-density polyethylene (HDPE),have particular applications in the manufacture of blow-molded andinjection-molded goods, such as food and beverage containers, film, andplastic pipe. Other types of polyolefins, such as low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), isotacticpolypropylene (iPP), and syndiotactic polypropylene (sPP) are alsosuited for similar applications. The mechanical requirements of theapplication, such as tensile strength and density, and/or the chemicalrequirements, such thermal stability, molecular weight, and chemicalreactivity, typically determine what type of polyolefin is suitable.

One benefit of polyolefin construction, as may be deduced from the listof uses above, is that it is generally non-reactive with goods orproducts with which it is in contact. This allows polyolefin products tobe used in residential, commercial, and industrial contexts, includingfood and beverage storage and transportation, consumer electronics,agriculture, shipping, and vehicular construction. The wide variety ofresidential, commercial, and industrial uses for polyolefins hastranslated into a substantial demand for raw polyolefin, which can beextruded, injected, blown or otherwise formed into a final consumableproduct or component.

To satisfy this demand, various processes exist by which olefins may bepolymerized to form polyolefins. These processes may be performed at ornear petrochemical facilities, which provide ready access to theshort-chain olefin molecules (monomers and comonomers), such asethylene, propylene, butene, pentene, hexene, octene, decene, and otherbuilding blocks of the much longer polyolefin polymers. These monomersand comonomers may be polymerized in a liquid-phase polymerizationreactor and/or gas-phase polymerization reactor to form a productcomprising polymer (polyolefin) solid particulates, typically calledfluff or granules. The fluff may possess one or more melt, physical,rheological, and/or mechanical properties of interest, such as density,melt index (MI), melt flow rate (MFR), copolymer content, comonomercontent, modulus, and crystallinity. The reaction conditions within thereactor, such as temperature, pressure, chemical concentrations, polymerproduction rate, and so forth, may be selected to achieve the desiredfluff properties.

In addition to the one or more olefin monomers, a catalyst forfacilitating the polymerization of the monomers may be added to thereactor. For example, the catalyst may be a particle added via a reactorfeed stream and, once added, suspended in the fluid medium within thereactor. An example of such a catalyst is a chromium oxide containinghexavalent chromium on a silica support.

Further, a diluent may be introduced into the polyolefin reactor. Thediluent may be an inert solvent and/or inert hydrocarbon, such asisobutane, propane, n-pentane, i-pentane, neopentane, and n-hexane,which is liquid at reaction conditions. However, some polymerizationprocesses may not employ a separate diluent, such as in the case ofselected examples of polypropylene production where the propylenemonomer itself acts as the diluent. In general, the diluent mayfacilitate circulation of the polymer slurry in the reactor, heatremoval from the polymer slurry in the reactor, and so on.

The slurry discharge of the reactor typically includes the polymer fluffas well as non-polymer components, such as unreacted olefin monomer (andcomonomer), diluent, and so forth. In the case of polyethyleneproduction, the non-polymer components typically comprise primarilydiluent, such as isobutane, having a small amount of unreacted ethylene(e.g., 5 wt. %). This discharge stream is generally processed, such asby a diluent/monomer recovery system (e.g. flash vessel or separatorvessel, purge column, etc.) to separate the non-polymer components fromthe polymer fluff.

The recovered diluent, unreacted monomer, and other non-polymercomponents from the recovery system may be treated, such as by treatmentbeds and/or a fractionation system, and ultimately returned as purifiedor treated feed to the reactor. Some of the components may be flared orreturned to the supplier, such as to an olefin manufacturing plant orpetroleum refinery. As for the recovered polymer (solids), the polymermay be treated to deactivate residual catalyst, remove entrained ordissolved hydrocarbons, dry the polymer, and pelletize the polymer in anextruder, and so forth, before the polymer is sent to customer.

The competitive business of polyolefin production drives manufacturersin the continuous improvement of their processes in order to lowerproduction costs, to address environmental concerns, and so on. In anindustry where billions of pounds of polyolefin product are produced peryear, small incremental improvements can result in significant economicbenefit, environmental progress, and so forth.

A particular issue in polyolefin production is the undesirable carryoverof volatile organics (e.g., diluent, monomer, and other hydrocarbons)with and in the polymer fluff particles exiting the “wet end” of thepolyolefin manufacturing process (e.g., exiting a purge column in themonomer/diluent recovery system). This stream of fluff particles exitingthe wet end is generally transferred to the “dry end” (e.g., through apneumatic conveying system) where the fluff may be stored in silos,extruded into pellets, and then loaded as pellets into containers orrailcars, and the like. The hydrocarbon in the stream of polymer fluffexiting the wet end (e.g., exiting a purge column) may be entrained withthe polymer fluff, dissolved in the polymer fluff, absorbed and/oradsorbed on the polymer fluff, contained within pores of the polymerfluff, and so on. The hydrocarbon may be undesirably released to theatmosphere at various points in the dry end process (e.g., in theextrusion and product loadout systems), resulting in the loss ofhydrocarbon, exceeding of environmental permitting allowances, and soon.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1 is a block flow diagram depicting an exemplary polyolefinmanufacturing system for producing polyolefins in accordance with oneembodiment of the present techniques;

FIG. 2 is a process flow diagram of an exemplary feed system of thepolyolefin manufacturing system of FIG. 1 in accordance with oneembodiment of the present techniques;

FIG. 3 is a process flow diagram of an exemplary catalyst preparationsystem of the feed system of FIG. 2 in accordance with one embodiment ofthe present techniques;

FIG. 4 is a process flow diagram of an exemplary catalyst activationsystem in accordance with one embodiment of the present techniques;

FIG. 5 is a process flow diagram of an exemplary reactor system and adiluent/monomer recovery system of the polyolefin manufacturing systemof FIG. 1 in accordance with one embodiment of the present techniques;

FIG. 6 is a diagrammatical representation of the exemplarypolymerization reactor of FIG. 5 showing the flow of cooling mediumthrough the reactor jackets in accordance with one embodiment of thepresent techniques;

FIG. 7 is a diagrammatical representation of an exemplary continuoustakeoff discharge of the polymerization reactor of FIG. 5 in accordancewith one embodiment of the present techniques;

FIG. 8 is a cross section along line 8-8 of FIG. 7 showing a ram valvearrangement in the continuous take off discharge assembly in accordancewith one embodiment of the present techniques;

FIG. 9 is a diagrammatical representation of a tangential location forthe continuous take off assembly in accordance with one embodiment ofthe present techniques;

FIG. 10 is a process flow diagram of the extrusion/loadout system ofFIG. 1 in accordance with one embodiment of the present techniques;

FIG. 11 is an exemplary plot of VOC (part per million or ppm) in thepolymer fluff stream exiting a purge column versus the polymer flufftemperature (° F.) in the purge column in accordance with one embodimentof the present techniques;

FIG. 12 is an exemplary plot of VOC (ppm) in the polymer fluff streamexiting a purge column versus the purge time (or residence time) inminutes in the purge column in accordance with one embodiment of thepresent techniques;

FIG. 13 is an exemplary plot of VOC (ppm) in the polymer fluff streamexiting a purge column versus the ratio of entering purge gas to polymerfluff (pound of gas per pound of polyethylene) in accordance with oneembodiment of the present techniques;

FIG. 14 is an exemplary plot of VOC (ppm) in the polymer fluff streamexiting a purge column versus the average particle size in microns (μm)of the polymer fluff in the purge column in accordance with oneembodiment of the present techniques;

FIG. 15 is an exemplary plot of VOC (ppm) in the polymer fluff streamexiting a purge column versus the pressure (pounds per square inch orpsig) of the purge gas (primarily nitrogen) entering the purge column inaccordance with one embodiment of the present techniques;

FIG. 16 is an exemplary plot of purge gas temperature (° F.) versus thepurge time (minutes) in a purge column in accordance with one embodimentof the present techniques;

FIG. 17 is a model process schematic of an the diluent/monomer recoverysystem depicted in FIG. 5 in accordance with one embodiment of thepresent techniques;

FIG. 18 is a material balance schematic for a an exemplary purge columnin accordance with one embodiment of the present techniques; and

FIG. 19 is a schematic of an exemplary interface for a purge columnmodel in accordance with one embodiment of the present techniques.

DETAILED DESCRIPTION OF THE EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

The present techniques provide for a model of a purge column in apolyolefin manufacturing process. Embodiments of the purge column modelmay be employed to design the purge column, as well as improve thedesign of upstream and downstream systems as a function of purge columnperformance. The purge column model may also be employed to manage andimprove the operation of the purge column including providing insight onoperating conditions of reducing the VOC of the polymer stream exitingthe purge column

It should be noted though the discussion at times may focus on theproduction of polyethylene and its copolymers, the disclosed techniquesafford benefits in the design and operation of purge columns or purgeseparators in the production of other polyolefins, such aspolypropylene, polybutylene, and so on. Finally, it should be apparentthat the various techniques may be implemented in a multiplicity ofcombinations.

I. Introduction

As discussed, polyolefin processes generally employ hydrocarbon(diluent, monomer, etc.) in the production of the polyolefin polymer.For example, a diluent (e.g., inert hydrocarbon solvent such asisobutane) may be used as carrier for the polymer in the reactor (loopreactor, gas phase reactor) to facilitate circulation (or bedfluidization) and heat removal in the reactor. The inert solvent andother hydrocarbons (e.g., monomer such as ethylene) are entrained ordissolved in the polymer, and are typically purged from the polymer in apurge separator or purge column with an inert gas (e.g., nitrogen). Thispurging system reduces carryover of hydrocarbons (VOC's) in the raw orvirgin polymer (i.e., fluff, flake, etc.) that was generated in thereactor (prior to being extruded into pellets, for example). Forenvironmental and economic reasons, the purged hydrocarbons, purge gas(e.g., nitrogen) are recovered and reused.

In an exemplary polyethylene production system utilizing one or moreloop reactors, a diluent isobutane is used as the carrier in thereactor. The isobutane is purged from the polymer in a purge column withnitrogen to tolerable hydrocarbon or VOC (volatile organic compound)limits. As discussed below, the nitrogen-rich gas having hydrocarbondischarging from the purge column is sent to an isobutane-nitrogenrecovery unit (INRU) for the recovery and reuse of the hydrocarbons andnitrogen. Embodiments of the present techniques facilitate the designand operation of the purge column, and in particular, in the design andoperation to reduce the VOC content of the polyolefin stream exiting thepurge column. Such polymer may then be sent to an extrusion and/orproduct loadout area, for example. It has been generally determined orconfirmed via the present techniques that factors affecting the VOC ofthe polymer stream exiting the purge column include, for example, flufftemperature, purge time, nitrogen flow and purity, fluff particle size,purge pressure, and resin density (or amorphous fraction), and so on. Itshould be emphasized that the present techniques may also be applicableto other polyolefin systems including those that employ other types ofliquid-phase polymerization reactors and also gas-phase reactors.

The present invention provides for one or more models and othertechniques for designing, rating, operating, and the like, a polyolefinpurge column (or separator) and for evaluating the impact of processparameters on purge column performance, for example. Embodiments of themodel(s) may be based on mass transfer theory and generally predict thehydrocarbon VOC for polymer stream leaving the purge column. In oneexample, the model is written in a Visual Basic Application program withan Excel worksheet as interface for model input and output. It should benoted that embodiments of the model have been validated with plant datafrom existing polyolefin processes (polyethylene and their copolymers).The validated model may be used to establish an approximate order ofsignificance of the process variables that influence degassing ofpolymer fluff in the purge column.

In certain examples for a given resin density, the variables forreducing VOC in the polymer stream leaving the purge column, indecreasing order of effectiveness, may be as follows: purge columntemperature, residence time or bed level of polymer in purge column;flow rate and hydrocarbon purity of purge nitrogen; particle size ofpolymer fluff; and operating pressure of purge column. Plant engineersand operators can rely on this exemplary list in selecting effectivevariables for reducing VOC in the polymer stream leaving the purgecolumn and nitrogen waste in the purge column, as well.

Lastly, it should be noted that the present techniques including thepurge column models also provide insight and improvement of processesupstream and downstream of the purge column. For example, the operationof the upstream catalyst system and/or reactor system may be adjusted inan effort to generate a desirable particle size distribution, asindicated by purge column model. In another example, the design and/oroperation of the flash line coupled to the discharge of the loop reactormay be adjusted. In this particular example, the present techniquesprovides for placement of vents on the steam lines to the flash line tofacilitate removal of non-condensable components

II. Polyolefin Production Process—an Overview

In the production of polyolefin, the polymerization reactor(s), whichpolymerize monomer into polyolefin, and the extruder(s), which convertthe polyolefin into polyolefin pellets, are typically continuousoperations. However, a variety of both continuous and batch systems maybe employed throughout the polyolefin process. An exemplary nominalcapacity for a typical polyolefin plant is about 600-800 million poundsof polyolefin produced per year. Exemplary hourly design rates areapproximately 85,000 to 90,000 pounds of polymerized polyolefin perhour, and 90,000 to 95,000 pounds of extruded polyolefin per hour.However, it should be emphasized that the foregoing numerical values areonly given as examples. Moreover, it should also be emphasized that thefollowing discussion of the exemplary manufacturing process 10 is notmeant to limit the applicability of the present purge column model andassociated techniques.

A. Feedstocks

Turning now to the drawings, and referring initially to FIG. 1, a blockdiagram depicts an exemplary manufacturing process 10 for producingpolyolefins, such as polyethylene homopolymer, polypropylenehomopolymer, and/or their copolymers. Various suppliers 12 may providereactor feedstocks 14 to the manufacturing system 10 via pipelines,trucks, cylinders, drums, and so forth. The suppliers 12 may compriseoff-site and/or on-site facilities, including olefin plants, refineries,catalyst plants, and the like. Examples of possible feedstocks 14include olefin monomers and comonomers (such as ethylene, propylene,butene, hexene, octene, and decene), diluents (such as propane,isobutane, n-hexane, and n-heptane), chain transfer agents (such ashydrogen), catalysts (such as Ziegler catalysts, Ziegler-Nattacatalysts, chromium catalysts, and metallocene catalysts), co-catalysts(such as triethylaluminum alkyl, triethylboron, and methyl aluminoxane),and other additives. In the case of ethylene monomer, exemplary ethylenefeedstock may be supplied via pipeline at approximately 800-1450 poundsper square inch gauge (psig) at 45-65° F. Exemplary hydrogen feedstockmay also be supplied via pipeline, but at approximately 900-1000 psig at90-110° F. Of course, a variety of supply conditions may exist forethylene, hydrogen, and other feedstocks 14.

B. Feed System

The suppliers 12 typically provide feedstocks 14 to a reactor feedsystem 16, where the feedstocks 14 may be stored, such as in monomerstorage and feed tanks, diluent vessels, catalyst tanks, co-catalystcylinders and tanks, and so forth. In the system 16, the feedstocks 14may be treated or processed prior to their introduction as feed 18 intothe polymerization reactors. For example, feedstocks 14, such asmonomer, comonomer, and diluent, may be sent through treatment beds(e.g., molecular sieve beds, aluminum packing, etc.) to remove catalystpoisons. Such catalyst poisons may include, for example, water, oxygen,carbon monoxide, carbon dioxide, and organic compounds containingsulfur, oxygen, or halogens. The olefin monomer and comonomers may beliquid, gaseous, or a supercritical fluid, depending on the type ofreactor being fed. Also, it should be noted that typically only arelatively small amount of fresh make-up diluent as feedstock 14 isutilized, with a majority of the diluent fed to the polymerizationreactor recovered from the reactor effluent. The feed system 16 mayprepare or condition other feedstocks 14, such as catalysts, foraddition to the polymerization reactors. For example, a catalyst may beactivated and then mixed with diluent (e.g., isobutane or hexane) ormineral oil in catalyst preparation tanks.

Further, the feed system 16 typically provides for metering andcontrolling the addition rate of the feedstocks 14 into thepolymerization reactor to maintain the desired reactor stability and/orto achieve the desired polyolefin properties or production rate.Furthermore, in operation, the feed system 16 may also store, treat, andmeter recovered reactor effluent for recycle to the reactor. Indeed,operations in the feed system 16 generally receive both feedstock 14 andrecovered reactor effluent streams. In total, the feedstocks 14 andrecovered reactor effluent are processed in the feed system 16 and fedas feed streams 18 (e.g., streams of monomer, comonomer, diluent,catalysts, co-catalysts, hydrogen, additives, or combinations thereof)to the reactor system 20. It should be noted that the feed system 16 isa source of the volatile organic compounds in the polymer fluff exitingthe purge column. Moreover, the design and/or operation of the feedsystem may be adjusted in response to various results provided by thepresent purge column models.

C. Reactor System

The reactor system 20 may comprise one or more reactor vessels, such asliquid-phase or gas-phase reactors. The reactor system 20 may alsocomprise a combination of liquid and gas-phase reactors. If multiplereactors comprise the reactor system 20, the reactors may be arranged inseries, in parallel, or in any other suitable combination orconfiguration. In the polymerization reactor vessels, one or more olefinmonomers are polymerized to form a product comprising polymerparticulates, typically called fluff or granules. The fluff may possessone or more melt, physical, rheological, and/or mechanical properties ofinterest, such as density, melt index (MI), melt flow rate (MFR),copolymer or comonomer content, modulus, and crystallinity. The reactionconditions, such as temperature, pressure, flow rate, mechanicalagitation, product takeoff, component concentrations, polymer productionrate, and so forth, may be selected to achieve the desired fluffproperties.

In addition to the one or more olefin monomers, a catalyst thatfacilitates polymerization of the monomer is typically added to thereactor. The catalyst may be a particle suspended in the fluid mediumwithin the reactor. In general, Ziegler catalysts, Ziegler-Nattacatalysts, metallocenes, and other well-known polyolefin catalysts, aswell as co-catalysts, may be used. An example of such a catalyst is achromium oxide catalyst containing hexavalent chromium on a silicasupport.

Further, diluent may be fed into the reactor, typically a liquid-phasereactor. The diluent may be an inert hydrocarbon that is liquid atreaction conditions, such as isobutane, propane, n-pentane, i-pentane,neopentane, n-hexane, cyclohexane, cyclopentane, methylcyclopentane,ethylcyclohexane, and the like. The purpose of the diluent is generallyto suspend the catalyst particles and polymer within the reactor. Somepolymerization processes may not employ a separate diluent, such as inthe case of selected polypropylene production where the propylenemonomer itself may act as the diluent.

A motive device may be present within the reactor in the reactor system20. For example, within a liquid-phase reactor, such as a loop slurryreactor, an impeller may create a turbulent mixing zone within the fluidmedium. The impeller may be driven by a motor to propel the fluid mediumas well as any catalyst, polyolefin fluff, or other solid particulatessuspended within the fluid medium, through the closed loop of thereactor. Similarly, within a gas-phase reactor, such as a fluidized bedreactor or plug flow reactor, one or more paddles or stirrers may beused to mix the solid particles within the reactor. Moreover, the designand/or operation of the reactor system may be adjusted in response tovarious results provided by the present purge column models.

D. Diluent/Monomer Recovery, Treatment, and Recycle

The discharge 22 of the reactors within system 20 may include thepolymer fluff as well as non-polymer components, such as diluent,unreacted monomer/comonomer, and residual catalyst. The discharge 22 maybe subsequently processed, such as by a diluent/monomer recovery system24, to separate non-polymer components 26 (e.g., diluent and unreactedmonomer) from the polymer fluff 28. The untreated recovered non-polymercomponents 26 may be further processed, such as by a fractionationsystem 30, to remove undesirable heavy and light components.Fractionated product streams 32 may then be returned to the reactorsystem 20 via the feed system 16. On the other hand, the non-polymercomponents 26 may recycle more directly to the feed system 16 (asindicated by reference numeral 34), bypassing the fractionation system30. Indeed, in certain embodiments, up to 80-95% of the diluentdischarged from the reactor bypasses the fractionation system in routeto the polymerization reactor.

As for the fluff 28, it may be further processed within the recoverysystem 24 and in the extrusion/loadout system 36, to prepare it forshipment, typically as pellets 38, to customers 40. Although notillustrated, polymer granules intermediate in the recovery system 24 andtypically containing active residual catalyst may be returned to thereactor system 20 for further polymerization, such as in a differenttype of reactor or under different reaction conditions. Thepolymerization and diluent recovery portions of the polyolefinmanufacturing process 10 may be called the “wet” end 42 or “reaction”side of the process 10, and the extrusion/loadout 36 portion of thepolyolefin process 10 may be called the “dry” end 44 or “finishing” sideof the polyolefin process 10.

E. Extrusion/Loadout System

In the extrusion/loadout systems 36, the fluff 28 is typically extrudedto produce polymer pellets 38 with the desired mechanical, physical, andmelt characteristics. Extruder feed may comprise additives, such as UVinhibitors and peroxides, which are added to the fluff products 28 toimpart desired characteristics to the extruded polymer pellets 32. Anextruder/pelletizer receives the extruder feed, comprising one or morefluff products 28 and whatever additives have been added. Theextruder/pelletizer heats and melts the extruder feed which then may beextruded (e.g., via a twin screw extruder) through a pelletizer dieunder pressure to form polyolefin pellets. Such pellets are typicallycooled in a water system disposed at or near the discharge of thepelletizer. An exemplary energy-saving technique includes the use of apellet water pump (e.g., having a 15-50 horsepower motor) to transportthe extruder pellets in the pellet water to the loadout area. This iscontrast to traditional approach of employing a conventional conveyingloop, which typically uses a pellet blower operating at about 250-500horsepower.

In general, the polyolefin pellets may then be transported to a productload-out area where the pellets may be stored, blended with otherpellets, and/or loaded into railcars, trucks, bags, and so forth, fordistribution to customers 40. In the case of polyethylene, pellets 38shipped to customers 40 may include low density polyethylene (LDPE),linear low density polyethylene (LLDPE), medium density polyethylene(MDPE), high density polyethylene (HDPE), and enhanced polyethylene. Thevarious types and grades of polyethylene pellets 38 may be marketed, forexample, under the brand names Marlex® polyethylene or MarFlex™polyethylene of Chevron-Phillips Chemical Company, LP, of The Woodlands,Tex., USA.

F. Customers, Applications, and End-Uses

Polyolefin (e.g., polyethylene) pellets 38 may be used in themanufacturing of a variety of products, components, household items andother items, including adhesives (e.g., hot-melt adhesive applications),electrical wire and cable, agricultural films, shrink film, stretchfilm, food packaging films, flexible food packaging, milk containers,frozen-food packaging, trash and can liners, grocery bags, heavy-dutysacks, plastic bottles, safety equipment, coatings, toys and an array ofcontainers and plastic products. Further, it should be emphasized thatpolyolefins other than polyethylene, such as polypropylene, may formsuch components and products via the processes discussed below.

Ultimately, the products and components formed from polyolefin (e.g.,polyethylene) pellets 38 may be further processed and assembled fordistribution and sale to the consumer. For example, a polyethylene milkbottle may be filled with milk for distribution to the consumer, or thefuel tank may be assembled into an automobile for distribution and saleto the consumer.

To form end-products or components from the pellets 38, the pellets aregenerally subjected to further processing, such as blow molding,injection molding, rotational molding, blown film, cast film, extrusion(e.g., sheet extrusion, pipe and corrugated extrusion,coating/lamination extrusion, etc.), and so on. Blow molding is aprocess used for producing hollow plastic parts. The process typicallyemploys blow molding equipment, such as reciprocating screw machines,accumulator head machines, and so on. The blow molding process may betailored to meet the customer's needs, and to manufacture productsranging from the plastic milk bottles to the automotive fuel tanksmentioned above. Similarly, in injection molding, products andcomponents may be molded for a wide range of applications, includingcontainers, food and chemical packaging, toys, automotive, crates, capsand closures, to name a few.

Extrusion processes may also be used. Polyethylene pipe, for example,may be extruded from polyethylene pellet resins and used in anassortment of applications due to its chemical resistance, relative easeof installation, durability and cost advantages, and the like. Indeed,plastic polyethylene piping has achieved significant use for watermains, gas distribution, storm and sanitary sewers, interior plumbing,electrical conduits, power and communications ducts, chilled waterpiping, well casing, to name a few applications. In particular,high-density polyethylene (HDPE), which generally constitutes thelargest volume of the polyolefin group of plastics used for pipe, istough, abrasion-resistant and flexible (even at subfreezingtemperatures). Furthermore, HDPE pipe may be used in small diametertubing and in pipe up to more than 8 feet in diameter. In general,polyethylene pellets (resins) may be supplied for the pressure pipingmarkets, such as in natural gas distribution, and for the non-pressurepiping markets, such as for conduit and corrugated piping.

Rotational molding is a high-temperature, low-pressure process used toform hollow parts through the application of heat to biaxially-rotatedmolds. Polyethylene pellet resins generally applicable in this processare those resins that flow together in the absence of pressure whenmelted to form a bubble-free part. Pellets 38, such as certain Marlex®HDPE and MDPE resins, offer such flow characteristics, as well as a wideprocessing window. Furthermore, these polyethylene resins suitable forrotational molding may exhibit desirable low-temperature impactstrength, good load-bearing properties, and good ultraviolet (UV)stability. Accordingly, applications for rotationally-molded Marlex®resins include agricultural tanks, industrial chemical tanks, potablewater storage tanks, industrial waste containers, recreationalequipment, marine products, plus many more.

Sheet extrusion is a technique for making flat plastic sheets from avariety of pellet 38 resins. The relatively thin gauge sheets aregenerally thermoformed into packaging applications such as drink cups,deli containers, produce trays, baby wipe containers, and margarinetubs. Other markets for sheet extrusion of polyolefin include those thatutilize relatively thicker sheets for industrial and recreationalapplications, such as truck bed liners, pallets, automotive dunnage,playground equipment, and boats. A third use for extruded sheet, forexample, is in geomembranes, where flat-sheet polyethylene material iswelded into large containment systems for mining applications andmunicipal waste disposal.

The blown film process is a relatively diverse conversion system usedfor polyethylene. The American Society for Testing and Materials (ASTM)defines films as less than 0.254 millimeter (10 mils) in thickness.However, the blown film process can produce materials as thick as 0.5millimeter (20 mils), and higher. Furthermore, blow molding inconjunction with monolayer and/or multilayer coextrusion technologieslay the groundwork for several applications. Advantageous properties ofthe blow molding products may include clarity, strength, tearability,optical properties, and toughness, to name a few. Applications mayinclude food and retail packaging, industrial packaging, andnon-packaging applications, such as agricultural films, hygiene film,and so forth.

The cast film process may differ from the blown film process through thefast quench and virtual unidirectional orientation capabilities. Thesecharacteristics allow a cast film line, for example, to operate athigher production rates while producing beneficial optics. Applicationsin food and retail packaging take advantage of these strengths. Finally,polyolefin pellets may also be supplied for the extrusion coating andlamination industry.

III. Polymerization Reactor Feed System

Referring to FIG. 2, a process flow diagram of an exemplary reactor feedsystem 16 (of FIG. 1) is depicted. In this embodiment, monomer 50 (e.g.,ethylene) is fed through monomer treaters 52 to the liquid phase reactor(e.g., loop slurry reactor) in the reactor system 20. Furthermore, amass flow meter 53, instead of an orifice plate meter, may be used tomeasure the flow rate of ethylene to the reactor. Indeed, the flow rateof ethylene monomer 50 to the reactor generally is typically measured(and controlled) to facilitate desired operating conditions (e.g.,slurry density, comonomer/monomer ratio, production rate, etc.) in thereactor and to provide the desired properties of the polyethylene formedin the reactor. The exemplary mass flow meter 53 used to measure theethylene monomer flow may be an orifice-plate type differential pressuremeter, Coriolis mass meter, and so forth.

Recycle diluent 54 (e.g., isobutane) with a relatively small amount ofentrained monomer may be returned from the diluent/monomer recoverysystem 24 (e.g., corresponding to stream 34 of FIG. 1) and sent to thepolymerization reactor. In the example of “direct” recycle to thereactor, the recycled diluent 54 may be cooled and passed through aheavies knockout pot 56, where heavy components are removed out of abottom discharge and sent via a centrifugal pump 58, for example, asfeed 60 to the fractionation system 30. The overhead 62 of the knockoutpot 56 may be further cooled in a heat exchanger 66 and collected in arecycle diluent surge tank 68 for feed to the reactor. Downstream, acentrifugal pump 70 may deliver the diluent 72 through recycle diluenttreaters 74 to the loop slurry reactor. It should be noted that arelatively small amount of fresh diluent (not illustrated) may be addedin the fractionation system 30, for example, to make-up for diluentlosses in the manufacturing process 10. Furthermore, comonomer 76 (e.g.,1-hexene) may be added to the suction of pump 70 or at other points inthe recycle diluent circuit for addition to the reactor. The monomertreaters 52 and recycle diluent treaters 58 may include molecular sieveor aluminum packing, for example, configured to remove catalyst poisonsfrom the monomer, recycle diluent, comonomer feeds, and other feeds.

Other feed components may be added to the loop slurry reactor. Forexample, hydrogen 60 may be added to control the molecular weight of thepolyolefin formed in the reactor. Furthermore, other additives, such asantistatic materials, may be injected into the reactor, as indicated byreference numeral 78. The various component streams may combine into asingle feed stream 80 for feed to the loop slurry reactor. Further, asdiscussed below, diluent 82 that is substantially olefin-free may berecycled from the fractionation system 30 through treaters 84 for use inthe preparation of the catalyst fed to the reactor. Indeed, diluent 82may act as a carrier of the catalyst stream 88 discharged from thecatalyst preparation system 86 in route to the loop slurry reactor.Lastly, treaters have been traditionally employed to process the variousfeeds, such as to remove catalyst poisons from the comonomer, freshisobutane, and hydrogen.

Referring to FIG. 3, a process flow diagram of the catalyst preparationarea 86 is depicted. A catalyst mix tank 140 receives catalyst 142, forexample, from a portable container. Olefin-free monomer 82 mixes withthe catalyst in the catalyst mix tank 140. An agitator 144 having amotor and drive 146 and blade 148 may facilitate mixing of the diluent82 and the catalyst 142 in the mix tank 140. The process catalyst 150discharges from the mix tank 140 and may enter, for example, a catalystrun tank 152 for metering to the loop slurry reactor. The run tank 152may also have an agitator 154 having a motor/drive 156 and agitatorblade 158 to maintain the catalyst mixed with the diluent. The catalystmay be metered, for example, by a positive displacement pump 160 to theloop slurry reactor as feed stream 88. Additionally, additives, such asco-catalysts (e.g., triethylaluminum) 162, may be added to the catalyst88 fed to the reactor. Finally, it should be noted that prior to mixingand metering the catalyst, the catalyst may be activated. For example,in the case of a chromium oxide catalyst, a catalyst activator mayconvert the chromium Cr3+ to Cr6+ for injection into the polymerizationreactor. While in the reactor and in contact with the ethylene monomer,for example, the chromium Cr6+ may reduce to Cr2+. Moreover, the designand/or operation of the catalyst system may be adjusted in response tovarious results provided by the present purge column models.

Referring to FIG. 4, a process flow diagram of a catalyst activatorsystem 170 is depicted. The activated catalyst product of system 170 isfed to the catalyst mix tank 140 (catalyst 142) of FIG. 3. In FIG. 4,the catalyst activator includes an internal vessel 172 containing thecatalyst, and an external furnace 174. Catalyst from the supplier may beheld in a holding vessel 176 and fed via to the internal vessel via anon/off valve 178, for example. Fuel 180 may be added via a sparger orpilot 182, for example, into the furnace 180, and the fuel 180 may becombined with air 184 injected into the furnace via an air filter 186and air blower 188. Combustion may take place inside the furnace in theregion 190, for example. The region 192 surrounding the internal vessel172 may experience operating temperatures in an exemplary range of 800to 1700° F. The heated fluid from this region 192 may discharge to theatmosphere 194, as depicted by arrow 196.

In addition to high heat, oxygen may be supplied to activate thecatalyst. Air 198 may be injected into the bottom of the internal vessel172 to provide the presence of oxygen inside the vessel, with heatprovided by the surrounding furnace 174. The air entering the vessel 172may exit at the top via an internal air filter 200, for example, andthen discharge to the atmosphere, as indicated by reference numeral 202.The activated catalyst may discharge from vessel 172 into a catalysttote bin 206, or other container. Furthermore, nitrogen 208 mayfacilitate discharge of the activated catalyst into the tote bin 206,and also provide an inert atmosphere in the tote bin 206. In general,catalyst activation processes include passing dry air through a catalystbed at a constant rate, while applying heat, until the catalyst reachesthe desired temperature, at which point the catalyst is held at theactivation temperature for the proper length of time. Moreover, thedesign and/or operation of the catalyst activation system may beadjusted in response to various results provided by the present purgecolumn models.

IV. Polymerization Reactor System

Referring to FIG. 5, a process flow diagram of an exemplarypolymerization reactor system 20 (of FIG. 1) and diluent/monomerrecovery system 24 (also of FIG. 1) are depicted. As discussed above,the reactor system 20 may comprise one or more polymerization reactors,which may in turn be of the same or different types. Furthermore, inmultiple reactor systems, the reactors may be arranged serially or inparallel. Whatever the reactor types comprising the reactor system 20, apolyolefin particulate product, generically referred to as “fluff”herein, is produced. To facilitate explanation, the following examplesare limited in scope to specific reactor types believed to be familiarto those skilled in the art and to single reactors or simplecombinations. To one of ordinary skill in the art using this disclosure,however, the present techniques are simply and easily applicable to morecomplex reactor arrangements, such as those involving additionalreactors, different reactor types, and/or alternative ordering of thereactors or reactor types. Such arrangements are considered to be wellwithin the scope of the present invention.

One reactor type comprises reactors within which polymerization occurswithin a liquid phase. Examples of such liquid phase reactors includeautoclaves, boiling liquid-pool reactors, loop slurry reactors (verticalor horizontal), and so forth. For simplicity, a loop slurry reactor 42that produces polyolefin, such as polyethylene, polypropylene, and theircopolymers, will be discussed in the context of the present techniquesthough it is to be understood that the present techniques are similarlyapplicable to other types of liquid phase reactors.

The loop slurry reactor 210 is generally composed of segments of pipeconnected by smooth bends or elbows. An exemplary reactor 210configuration includes eight jacketed vertical pipe legs, approximately24 inches in diameter and approximately 200 feet in length, connected bypipe elbows at the top and bottom of the legs. As discussed below,reactor jackets 212 are normally provided to remove heat from theexothermic polymerization via circulation of a cooling medium, such astreated water, through the reactor jackets 212.

The reactor 210 may be used to carry out polyolefin polymerization underslurry conditions in which insoluble particles of polyolefin are formedin a fluid medium and are suspended as slurry until removed. A motivedevice, such as pump 214, circulates the fluid slurry in the reactor210. An example of a pump 214 is an in-line axial flow pump with thepump impeller disposed within the interior of the reactor 210 to createa turbulent mixing zone within the fluid medium. The impeller may alsoassist in propelling the fluid medium through the closed loop of thereactor at sufficient speed to keep solid particulates, such as thecatalyst or polyolefin product, suspended within the fluid medium. Theimpeller may be driven by a motor 216 or other motive force.

The fluid medium within the reactor 210 may include olefin monomers andcomonomers, diluent, co-catalysts (e.g., alkyls, triethylboron, methylaluminoxane, etc.), molecular weight control agents (e.g., hydrogen),and any other desired co-reactants or additives. Such olefin monomersand comonomers are generally 1-olefins having up to 10 carbon atoms permolecule and typically no branching nearer the double bond than the4-position. Examples of monomers and comonomers include ethylene,propylene, butene, 1-pentene, 1-hexene, 1-octene, and 1-decene. Again,typical diluents are hydrocarbons which are inert and liquid underreaction conditions, and include, for example, isobutane, propane,n-pentane, i-pentane, neopentane, n-hexane, cyclohexane, cyclopentane,methylcyclopentane, ethylcyclohexane, and the like. These components areadded to the reactor interior via inlets or conduits at specifiedlocations, such as depicted at feed stream 80, which generallycorresponds to one of the feed streams 18 of FIG. 1. Likewise, acatalyst, such as those previously discussed, may be added to thereactor 210 via a conduit at a suitable location, such as depicted atfeed stream 88, which may include a diluent carrier and which alsogenerally corresponds to one of the feed streams 18 of FIG. 1. In total,the added components generally compose a fluid medium within the reactor210 within which the catalyst is a suspended particle.

The reaction conditions, such as temperature, pressure, and reactantconcentrations, are regulated to facilitate the desired properties andproduction rate of the polyolefin in the reactor, to control stabilityof the reactor, and the like. Temperature is typically maintained belowthat level at which the polymer product would go into solution. Asindicated, due to the exothermic nature of the polymerization reaction,a cooling fluid may be circulated through jackets 212 around portions ofthe loop slurry reactor 210 to remove excess heat, thereby maintainingthe temperature within the desired range, generally between 150° F. to250° F. (65° C. to 121° C.). Likewise, pressure may be regulated withina desired pressure range, generally 100 to 800 psig, with a range of450-700 psig being typical.

As the polymerization reaction proceeds within the reactor 210, themonomer (e.g., ethylene) and comonomers (e.g., 1-hexene) polymerize toform polyolefin (e.g., polyethylene) polymers that are substantiallyinsoluble in the fluid medium at the reaction temperature, therebyforming a slurry of solid particulates within the medium. These solidpolyolefin particulates may be removed from the reactor 210 via asettling leg or other means, such as a continuous take-off, as depicteddischarge stream 22. In downstream processing, the polyethylenedischarged from the reactor may be extracted from the slurry andpurified.

FIG. 6 depicts an exemplary polymerization reactor 210 of FIG. 5 andshows a counter-current flow scheme of cooling medium through thereactor jackets 212A-H. Again, the loop reactor 210 is generallycomposed of segments of pipe connected by smooth bends or elbows. Amotive device, such as pump 214, circulates the fluid slurry in thereactor 210. An example of a pump 214 is an in-line axial flow pump withthe pump impeller disposed within the interior of the reactor 210. Acoolant system 250 removes heat from the loop reactor 210 via reactorjackets 212A-H. The coolant system 250 provides a coolant supply 252(e.g., treated water) and processes a coolant return 254.

As the polymerization reaction proceeds within the reactor 210, thereaction conditions may be controlled to facilitate the desired degreeof polymerization and the desired reaction speed while keeping thetemperature below that at which the polymer product would go intosolution. As mentioned, due to the exothermic nature of thepolymerization reaction, cooling jackets 212A-H may be provided aroundportions of the closed loop system through which a cooling fluid iscirculated as needed to remove excess heat (heat of reaction), therebymaintaining the temperature within the desired range, generally between150° F. to 250° F. (65° C. to 121° C.).

In general, reactor temperature varies linearly with changes in thereactor system operating conditions. An accepted assumption in the artis that heat generated in the reactor by the exothermic polymerizationis linear with the polyolefin production rate (i.e., pounds per hour ofpolyolefin polymerized). Thus, reactor temperature, which is anindication of the energy or heat in the reactor, varies linearly withproduction rate. As appreciated by those of ordinary skill in the art,typical reactor temperature control may involve aproportional-integral-derivative (PID) algorithm.

V. Diluent/Monomer Recovery System

A. Flash Chamber

Returning to FIG. 5, the discharge 22 from the reactor 210 may flowthrough an in-line flash heater 222 and into a flash chamber 224. Thein-line flash heater 222 may be a surrounding conduit that uses steam orsteam condensate, for example, as a heating medium to provide indirectheating to the discharge 22. Thus, the loop slurry reactor 210 effluent(discharge 22) is heated prior to its introduction into the flashchamber 224. Also, before the discharge 22 enters the flash chamber 224,water or other catalysts poisons may be injected into the discharge 22to deactivate any residual catalysts in the discharge 22 stream. Becausethese injected components are catalysts poisons by definition, they aretypically completely removed, or at least substantially removed, fromany recovered material (e.g., monomer or diluent) recycled to thereactor 210.

In the flash chamber 224, most of the non-solid components of thereactor discharge 22 are withdrawn overhead as vapor in the flash gas226. Note, it is this recycled flash gas 226 that may bypass thefractionation system in route to the reactor 210 (i.e., via the feedsystem 16). In polyethylene production, this vapor is typicallyprimarily diluent, such as isobutane or other diluents previouslymentioned. It may also contain most of the unreacted monomer (e.g.,ethylene) and other light components, as well as unreacted comonomer(e.g., 1-hexene, butene, 1-pentene, 1-octene, and 1-decene) and otherheavy components (e.g., hexane and oligomers). In general lightcomponents or “lights” may be defined at those light components withlower boiling points than the diluent employed. In contrast heavycomponents or “heavies” may be defined as those components having higherboiling points than the diluent. An exemplary approximate composition ofthe flash gas 226 is 94 wt. % isobutane, 5 wt. % ethylene, and 1 wt. %other components. A level or volume of fluff may be maintained in theflash chamber 224 to give additional residence time of the fluff in thechamber 224 to facilitate separation of liquid and vapor entrained inthe porous fluff particles.

The flash gas 226 may be processed in equipment such as cyclones, bagfilters, etc., where entrained fluff solids are removed and returned tothe flash chamber 224 or to downstream equipment, such as the purgecolumn discussed below. The flash gas 226 may also travel through adeoxygenation bed, for example. Furthermore, the flash gas 226 may becooled or condensed in a heat exchanger (e.g., shell-and-tubeconstruction) prior to its recycle to the feed system 16 orfractionation system 30.

As for the solids (polymer) in the flash chamber 224, they are withdrawnwith a small amount of entrained diluent (and monomer) and sent to apurge column 228 via solids discharge 230. As will be appreciated bythose of ordinary skill in the art, the solids discharge 230 conduit mayinclude valve configurations that allow polymer to flow downward throughthe conduit while reducing the potential for vapor to flow between thepurge column 228 and the flash chamber 56. For example, one or morerotary or cycling valves may be disposed on the solids discharge 230conduit. Furthermore, a relatively small fluff chamber may also bedisposed on the conduit to handle the discharge of the fluff solids fromthe flash chamber 224 to the purge column 228. Such a discharge to thepurge column 228 may include appropriate valve configurations, a surgechamber, or simply a conduit, and so on. Moreover, other arrangements inthe flash/purge system are applicable. For example, the fluff solidsfrom the flash chamber 224 may discharge to a lower pressure flashchamber (with the lower pressure flash gas compressed for recycle tofractionation system 30 and reactor) prior to introduction of the fluffsolids to the purge column 228.

Finally, as discussed, the system 10 may provide for direct recycle of80 to 95 wt. % of the diluent and unreacted monomer recovered from thein the monomer/recovery system 24 to the feed and reactor systems 16 and20. For example, flash gas 226 (FIG. 7) which discharges from the flashchamber 224 overhead, and which generally corresponds to the recyclestream 34 of FIG. 1, may be sent as the recycle diluent 54 stream (FIG.2) directly to the reactor 210 via the surge tank 68. Such directrecycle significantly reduces the load on the fractionation system 30,including the load on the fractionation columns and reboilers in thesystem 30 (as compared with no direct recycle). Thus, the fractionationcolumns and associated reboilers (e.g., steam reboilers) may besignificantly reduced in size (e.g., reduced by 80-95% of theconventional size) for the same capacity polyolefin plant. Steam usageis significantly reduced and substantial energy by employing the smallercolumns. Lastly, it should be noted that the design and/or operation ofthe fractionation system 30 may be adjusted in response to the output ofthe present purge column models.

B. Purge Column

The primary solids feed to the purge column 228 is typically the solidsdischarge 230 (polyolefin fluff) that exits the flash chamber 224. Apurpose of the purge column 228 is to remove residual hydrocarbon fromthe entering solids streams and to provide substantially-clean polymerfluff 232 with relatively small amounts of entrained volatile organiccontent (VOC). The fluff 232 may be transported or conveyed to theextrusion/loadout system 36 for conversion to pellets 38, and fordistribution and sale as polyolefin pellet resin to customers 40. Ingeneral, the treated polymer particles discharged from purge column 228as polymer fluff 232 may be processed in a conventional finishingoperation, such as a screw extruder, in the extrusion/load out system 36(FIG. 1).

In the exemplary purge column system illustrated, nitrogen is circulatedthrough purge column 228 to remove residual hydrocarbons via overheaddischarge 234. This discharge 234 may be sent through a separation unit236, such as a membrane recovery unit, pressure swing adsorption unit,refrigeration unit, and so forth, to recover nitrogen via nitrogenstream 238, and to discharge a separated hydrocarbon stream 240 as feedto the fractionation system 30. In the art, the separation unit 236 maybe known as an Isobutane Nitrogen Recovery Unit (INRU) or DiluentNitrogen Recovery System (DNRU). Moreover, fresh nitrogen 242 may beadded to the nitrogen circuit to account for nitrogen losses in thepurge column 228 system. Finally, it should be noted that thehydrocarbon stream 240 may beneficially provide feed to thefractionation system 30 (see FIG. 13). For example, the hydrocarbonstream 240 discharging from the separation unit 236 makes availablehydrocarbon feed that may be processed to give the olefin-free diluentused in catalyst preparation.

An exemplary purge column 228 may be cylindrical vessel having arelatively tall vertical section, a cover or head at the top, slopedsides or conical shape at the bottom with an opening for polymer fluffdischarge. The polymer fluff to be degassed of volatile hydrocarbons mayenter the vessel at the top, while the purge gas, typically nitrogen,may be introduced to the vessel in the slopped bottom sides. Flow may becountercurrent between the purge gas and polymer fluff in the vessel.Again, in certain embodiments, the hydrocarbon rich purge gas leaves thevessel through an opening at the top, while the degassed fluff leaves atthe bottom of the vessel.

Degassing effectiveness in the vessel may be predicated on themaintenance of uniform plug flow of the polymer fluff and purge gas inthe vessel, thereby ensuring good contact between the two. The diameterof the vessel typical range from 5 to 6 feet, but its length (L/D ratio)is chosen to achieve a residence time (e.g., 30 to 180 minutes)sufficient for degassing the polymer fluff. Example L/D ratios may rangefrom 4 to 8, or outside this range. Lastly, it should be noted thatinternals may be employed in the purge column, such as a distributorplate for introducing purge gas (nitrogen), an inverted cone forfacilitating plug glow of the polymer (e.g., reduce bridging orchanneling of the polymer fluff), and so on.

C. Alternate Configurations of the Diluent/Monomer Recovery System

As will be appreciated by those of ordinary skill in the art, a varietyof configurations may be employed in the diluent/monomer recovery system24. For example, the solids discharge 230 from the flash chamber 224 maybe sent to another reactor (e.g., a gas phase reactor) instead of to thepurge column 228 or to a low-pressure flash chamber. The polymer maythen later enter the purge column 228 (i.e., from the gas phase reactoror low-pressure flash chamber). If discharged to another reactor fromthe flash chamber 224, catalyst poison may not be injected upstream inthe discharge 22, and, thus, residual active catalysts remain forfurther polymerization. In another configuration, the purge column 228may be combined with a downstream extruder feed tank. The separationunit 236 associated with the purge column 228 may then accommodate thenew purge column/extruder feed tank combination, for example.

VI. Continuous Take Off of the Reactor Effluent Discharge

FIGS. 7-9 illustrate a continuous take-off mechanism of the reactordischarge 22. Referring to FIG. 7, a continuous takeoff mechanism 280disposed on a pipe elbow of the loop slurry reactor 210, is depicted.The continuous takeoff mechanism 280 includes a take-off cylinder 282, aslurry withdrawal line 284, an emergency shut-off valve 285,proportional motor valve 286 to regulate flow, and a flush line 287. Thereactor 210 may be operated “liquid” full, and because the reactorliquid contents are slightly compressible, pressure control of theliquid through the system may be accomplished with a valve. Further,where diluent input is held substantially constant, and the proportionalmotor valve 58 may be used to control the rate of continuous withdrawaland to maintain the total reactor pressure within designated set points.

Referring to FIG. 8, which is taken along section line 8-8 of FIG. 7, asmooth-curved pipe elbow having the continuous take-off mechanism 280,is depicted. Thus the illustrated pipe elbow may be considered amappendage-carrying elbow. As shown, the mechanism includes take-offcylinder 282, which is attached, in this instance, at a right angle to atangent to the outer surface of the elbow. Further, coupling to thecylinder 282 is the slurry withdrawal line 284. Disposed within the takeoff cylinder 282 is a ram valve 288, which may serve at least twopurposes. First, it may provide a clean-out mechanism for the take-offcylinder if it should ever become fouled with polymer, for example.Second, it may serve as a shut-off valve for the entire continuoustake-off assembly.

FIG. 9 shows an attachment orientation for the take-off cylinder 282,which is affixed tangentially to the curvature of the elbow and at apoint just prior to the slurry flow turning upward. The opening may beelliptical to the inside surface, for example, and further enlargementmay be implemented to improve solids take-off. Finally, it should benoted that a variety of orientations of the attachment of the take-offcylinder 282 may be implemented.

A continuous take-off of product slurry of an olefin polymerizationreaction carried out a loop reactor in the presence of an inert diluentallows operation of the reactor at a much higher solids concentrationthan with the conventional settling leg(s) used to discharge thepolymer. For example, production of predominantly ethylene polymers(polyethylene) in isobutane diluent has generally been limited to amaximum solids concentration in the reactor of 37-40 weight percent (wt.%) with the settling leg configuration. However, the continuous take-off(CTO) has been found to allow significant increases in solidsconcentration. As a result, solids concentration of greater than 50 wt.% in the reactor may be implemented with the continuous takeoff. Itshould be emphasized that in a commercial operation, as little as a onepercentage point increase in solids concentration is of majorsignificance. Such an increase, for example, allows higher productionrates of polyethylene.

VII. Extrusion/Loadout System

Referring to FIG. 10, a process flow diagram of the extrusion/loadoutsystem 36 of FIG. 1 is depicted. Polyolefin fluff 232 from the purgecolumn 228 (FIG. 5) may be pneumatically transferred, for example, usinga dilute phase blower, through a valve 340 in the extruder/loadoutsystem 36, and either into conduit 342 to the fluff silo 344, or intoconduit 346 to the extruder feed tank 348. The fluff silo 344 may beused to provide surge capacity during shutdown of the extruder (or ofother operations) in the extrusion/loadout system 36. On the other hand,the fluff silo 344 may also accumulate fluff to allow for full-rateoperation of the extruder while the upstream polymerization reactor 210“catches up” during start up of the reactor 210. The polyolefin fluff insilo 344 may be pneumatically transferred to the extruder feed tankthrough rotary valve 350 with the aid of a blower system 351.

Typically, however, the primary flow of polyolefin fluff 232 (which maygenerally correspond to fluff 28 of FIG. 1) is to the extruder feed tank348 via conduit 346. Downstream, rotary valve fluff 352 may feedpolyolefin fluff 354 to the extruder 356, where the extruder heats,melts, and pressurizes the polyolefin fluff 354. As will be appreciatedby those of ordinary skill in the art, the fluff 354 from the extruderfeed tank 348 may be metered to the extruder 356 with a variety ofmeters, such as smart flowmeter-type, master-feeder type, and so forth.Furthermore, additives may be injected into the fluff 354 stream at anaddition rate that may be based on a specified ratio to the mass flowrate of the fluff 354. This ratio or “slave” feed of additives to fluff354 may be specified at a value to generate a desired recipe, forexample, for each polyolefin grade or product, and to give the desiredproperties of the downstream polyolefin pellets. Furthermore, theadditive addition may be accomplished with a liquid additive system,loss-in-weight-feeders, and the like. In certain embodiments, one ormore of lost-in-weight feeders may be used to meter a pre-mixed additivepackage fed from a bulk container, for example, to the extruder 356 viathe fluff 354 stream, an extruder 354 feed hopper, directly to theextruder 354, and so on.

In general, the extruder 356 may melt, homogenize, and pump thepolyolefin polymer and additives through a pelletizer 358, which mayinclude a screen pack and heated die head, for example, which pelletizesthe mixture of fluff and additives. Further, pelletizer knife blades(i.e., under water) may cut the polyolefin melt extruded through the dieinto pellets. The pellets are typically quenched by water 360 and maytravel in a pellet-water slurry 362 from the pelletizer 358 to a pelletdewatering dryer 364. The dryer 364 may separate the free water and thendry the remaining surface water from the pellets by centrifugal force.The dried pellets 366 may discharge onto a scalping screen 368, forexample, which removes oversized and undersized pellets fromon-specification pellets 370.

Water 360 may be supplied to the pelletizer 358 from a water tank 372via a centrifugal pump 374 and cooler 376 (e.g., shell and tube heatexchanger). Water 378 removed from the pellet dryer 364 may return tothe water tank 372. The polyolefin pellets 370 exiting the scalpingscreen 368 may fall by gravity through a rotary valve 380 into adense-phase pneumatic conveying line 382, for example, and transportedto pellet silos 384. The pellet silos may include storage tanks,blenders, off-specification storage tanks, and so on. In the illustratedembodiment, the blower package 385 provides nitrogen and/or air 388 toconvey the pellets 370 via conveying line 382 to the pellet silos 386.Polyolefin pellets 390 may be loaded into rail cars 392, hopper cars,trucks, tote bins, bags, and so on. Pellets 390 may be loaded intohopper cars, for example, using a gravity type, air assisted,multiple-spout, loading system. Such a system may allow the hopper carto be automatically loaded at a rate higher than the polymerization andextrusion production rate. Thus, extra “time” generated by the higherloadout rates may be exploited to provide time to move the hopper carsor rail cars after filling, and to spot the next empty car. Lastly, itshould be noted that VOC in the polyolefin fluff stream 232 from thepurge column 228 (FIG. 5) may collect at various points in theextrusion/loadout system 36. Moreover, the VOC may escape or be ventedto the atmosphere from various points in the extrusion/loadout system36.

VIII. Purge Column Model

A. Exemplary Results

FIGS. 11-16 show example results of sensitivity studies with anexemplary model to evaluate the impact of purge parameters (e.g., flufftemperature, residence time, nitrogen flow, particle size, and pressure)on the VOC of polyolefin polymer (i.e., fluff) stream leaving the purgecolumn. The exemplary model is based on mass-transfer theory discussedin detail the following sections below. As expected, the VOC of thepolymer stream decreased for higher fluff temperature, longer residencetime, higher purge gas flow (e.g., inert gas, nitrogen, air, etc.),smaller polymer particle size, and lower purge pressure. It is believedthat these parameter trends generally enhance the diffusion of absorbedor adsorbed hydrocarbon components from the polymer. Highlights of theexemplary results are discussed below.

FIG. 11 is an exemplary plot 400 of model results of VOC 402 in ppm(part per million—volatile organics in the polyolefin fluff streamleaving the purge column) versus the temperature 404 of the polyolefin(fluff) in degrees Fahrenheit. The curve 406 illustrates an exampleinverse relationship between the VOC 402 and the fluff temperature 404.In this example, increasing polyolefin fluff temperature in the purgecolumn from 160° F. to 176° F. (10% change) resulted in about 80%reduction in VOC, from 128 ppm to 25 ppm, as depicted in the exemplaryplot 400 of FIG. 11.

FIG. 12 is an exemplary plot 410 of model results of VOC 412 in ppm inthe polyolefin (fluff) stream discharging from the purge column (e.g.,purge column 228) versus the purge time 414 in minutes. In thisembodiment, the purge time 414 is the residence time of the polyolefinfluff in the purge column. The curve 416 illustrates an example inverserelationship between VOC 412 and purge time 414. In this example,increasing purge time 414 from 60 minutes to 75 minutes (25% increase)resulted in about 73% reduction in VOC 412, from 34 ppm to 9 ppm.However, with this desired increase in purge time 414, production rateof polyolefin fluff discharging from the upstream polymerization reactor(e.g., loop reactor 210) was decreased by 20% in the model to accountfor inventory control in the purge column.

FIG. 13 is an exemplary plot 420 of model results of VOC 422 in thepolyolefin leaving the purge column versus the ratio 424 of the flowrate of purge gas (in this example, nitrogen) to the purge column to theflow rate of polyolefin polymer leaving the purge column. As with theaforementioned variables, the curve 426 illustrates an example inverserelationship of VOC 422 versus the ratio 424 of flow rates. In thisembodiment of the model, increasing purge gas from 694 pounds per hour(lb/hr) to 810 lb/hr (17% increase) resulted in about a 40% reduction inVOC from 50 ppm to 30 ppm. In this calculation, the residence time andfluff temperature in the purge column were maintained constant.

FIG. 14 is an exemplary plot 430 of model results of VOC 432 in thepolyolefin (fluff) stream discharged from the purge column versusaverage particle size 434 in 10 ⁻⁶ meters (microns or μm) the polyolefinpolymer (fluff). A proportional relationship exists between VOC 432 andpolymer particle size 434, as illustrated by exemplary curve 436. In onesensitivity example of the model, with reducing fluff particle size from800 microns to 600 microns (25% decrease), the VOC 432 was reduced byabout 43% from 47 ppm to 26 ppm.

FIG. 15 is an exemplary plot 440 of model results of VOC 442 in thepolyolefin (fluff) stream discharged from the purge column versus thepressure 444 of the purge gas (e.g., inert gas) entering the purgecolumn. The curve 446 illustrates an example proportional relationshipbetween VOC 442 and purge pressure 444. In the exemplary model, reducingpurge pressure 444 from 5 pounds per square inch (psig) to 3 psig, a 40%reduction, resulted in about a 36% reduction in VOC from 44 ppm to 28ppm.

FIG. 16 is a plot 450 of exemplary model results of purge temperature452 (i.e., temperature of the polyolefin in the purge column) versus thepurge time 454 (i.e. residence time of the polyolefin in the purgecolumn). The curve 456 represents the trade-off between purgetemperature 454 (fluff temperature) and purge time 454 (residence time),while maintaining the constant VOC in the polyolefin stream and theinventory in the purge column. In these example results of the purgecolumn model, an approximate 10% drop in purge (fluff) temperature from170° F. to 153° F. generally corresponds to a need for about a 50%increase in purge time (residence time) in the purge column from 59minutes to 88 minutes to maintain VOC (no increase) in the polyolefinstream discharging from the purge column.

Table 1 summarizes an exemplary order of the effects of the parameterson VOC reduction for an example resin in this case study. The exampleresin is grade TR-418F obtained from Chevron Phillips Chemical CompanyLP of The Woodlands, Tex. The resin TR-418F generally has a fluff bulkdensity of 0.90 to 0.97 pound per cubic feet. Ultimate applications ofsuch a resin may include film, pipe, blow-molding, and the like.Analysis with the resin is only given as an example, and not meant tolimit the present techniques to a particular resin or particular gradeof resin. Moreover, the present techniques and purge column model mayaccommodate a variety of unimodal or bimodal polyolefin polymers orresins, resins produced with a single reactor or multiple reactors inseries, and so on.

TABLE 1 Exemplary Impact/Order of purge parameter on VOC reduction forpolymer /ΔVOC/VOC/ Parameter (Par) ΔPar/Par ΔVOC/VOC ΔPar/Par|Temperature (F.) 1.4% −21.2% 15.0 Residence Time (min) 7.3% −30.2% 4.2Purge Gas Flow (lb/hr) 3.2% −8.7% 2.8 Particle Size (μm) 5.2% 13.3% 2.5Purge Pressure (psig) −15.9% −15.4% 1.0

In this example of TR-418F polyolefin resin, the approximate order ofsignificance in reducing VOC in the polyolefin polymer stream exitingthe purge column is fluff temperature, residence time, purge gas (e.g.,nitrogen) flow, fluff particle size, and purge pressure. In thisembodiment, high fluff temperature had the most impact on VOC reductionfor the polymer. Therefore, in this instance, the highest possible purgecolumn fluff temperature achieves the highest possible recovery ofhydrocarbons in the INRU and reduces potential VOC emissions in theplant. The highest possible purge column temperature may be generallyachieved by running at the highest possible flash gas temperature beforeencountering temperature-related operational problems (e.g., limited bythe softening point or melting point of the polyolefin). It should benoted that the flash gas temperature is generally controlled via theupstream flash line heaters. In this example, heating the purge gas hasrelatively little impact on degassing as the heat capacity of the purgegas (e.g., nitrogen) is typically low and the mass flow of the purge gascompared to the mass flow of the polymer is miniscule.

The reduced VOC observed for smaller fluff particle size suggests it maybe beneficial to employ catalyst or reactor technology for producingpolymer with smaller particle size. Apart from potential bettercirculation in the upstream polymerization reactor for such smallerparticle resins, the small particles may lead to better degassing in thepurge column for the resin, especially the low-density type. Theselected minimum or low end of the polymer particle size may bedetermined by needs of circulation in the reactor, settling efficiencyof the polyolefin fluff transfer systems.

In summary, the exemplary purge column models of the present techniquesmay present a significant contribution to the array of tools forsupporting polyolefin production. The model can be used for plantsupport and for evaluating purge column designs and operation. The modelis general and applies to the degassing of non-molten polyethyleneresins, generally irrespective of their technology of production (liquidphase polymerization gas-phase polymerization, loop reactor, autoclavereactors, fluidized bed reactors, stirred reactors, etc.).

B. Exemplary Process Schematic of Model

FIG. 17 is an exemplary process schematic of one embodiment of the purgecolumn model. FIG. 17 shows an exemplary diluent/monomer recovery system24 including the flash line heater 22, flash vessel 224, and purgecolumn 228. Hydrocarbon or diluent-rich polyolefin polymer slurry iswithdrawn from the upstream polymerization reactor 210 (e.g., via acontinuous take-off from the reactor 210). The slurry is heated in theflash line heater 222 with steam to boil-up the hydrocarbon liquid(e.g., diluent and unreacted monomer and comonomer) associated with thepolymer into vapor. Steam 458 enters a jacket (i.e., annulus of outerconduit over inner conduit) of the flash line heater 222 and exits assteam condensate 459. More than one continuous take-offs (and more thanone associated flash line heater 222) may be employed for a givenreactor 210.

The polymer-vapor mixture is flashed in the flash tank 224 to separateout the hydrocarbon vapor (flash gas 226) from the polymer fluff(discharged as polyolefin stream 230). As discussed, in this embodiment,stream 230 which includes polymer fluff and any entrained hydrocarbonare sent to the purge column 228 for purging with recycled purge gas(nitrogen) 238 (and fresh purge gas (nitrogen) 242) to reduce thehydrocarbon content (or volatile organic content) of the polyolefinpolymer to desirable or allowable limits. It should be noted that theflash gas 226 may be processed by a bag filter 464 (or cyclone) and theflash gas 226A then sent to a heavies column in the fractionation system30, for example. Any solid particles recovered via the bag filter 464may be sent to the purge column, for example. Likewise, the overheadhydrocarbon-rich purge gas 234 exiting the top of the purge column 228may be processed in a bag filter 466, where solids 470 (e.g., polymersolids) are removed and returned to the purge column 228.

The process configuration for the present models may incorporate adischarge of polyolefin polymer form the flash vessel 224 to travelthrough a rotary valve 460, as well as through other equipment, such asfluff surge chambers, etc. Also, the nitrogen entering the purge columnmay be heated in a heat exchanger 472 employing steam 474, for example.Further, the polyolefin fluff exiting the bottom portion of the purgecolumn may travel through one or more rotary valves 462 and 232 in theextrusion part of the plant.

As indicated, several factors may influence the effectiveness of thepurge column 228 in degassing the polymer, namely, purge temperature,purge time, purge gas (nitrogen) purity and flow rate, particle size,purge pressure, polymer density, and so on. The present techniquesprovide for one or more models for designing and rating purge columns228 to select the various mix of parameters for the effective purging ofpolymer resins. Again, while portions of the present discussion mayfocus on construction and use of a model for purge columns 228 in theloop slurry polyethylene plant, the model is readily applicable andadaptable to address the purging of polymer in general, such as in theloop polypropylene plants, gas-phase plants, and so forth.

C. Model Description

It is assumed in embodiments of the purge column model that the purgetime is the residence time (T_(res)) of polymer in the purge column.Thus, if the production rate of the plant is W_(PE), the inventory ofpolymer in the column (W_(inv)) is,

W _(inv) =W _(PE) *T _(res)  (1).

Then, given the bulk density of polymer (ρ_(b)) in the column and thediameter of the column (D_(bed)), the height of inventory in the column(H_(bed)) becomes,

H _(bed) =W _(inv)/(0.25πD _(bed) ²ρ_(b))  (2).

For an operating plant, the inventory of the purge column is generallyexpressed as a fraction (f_(bed)) of the operating window between theminimum inventory level (H_(LL)) and the maximum inventory level(H_(HL)) of the column. In which case the inventory level for the columnbecomes,

H _(bed) =H _(LL) +f _(bed)*(H _(HL) −H _(LL))  (3).

Therefore, the residence time and the material inventory for the purgecolumn can be estimated for an operating plant from equations 1-2.Having determined the residence time and inventory for the purge column,the next step in the model may be to estimate the height of a separationstage (HETP) and the number of stages (N) for the column,

N=H _(bed)/HETP  (4).

In one example, the method of Oanda and et al. (Kister, 1992) is used inthe model to estimate HETP for the column and its number of separationstages. FIG. 18 depicts the stage-wise material flow 480 having stages484 for a component i (e.g., a particular hydrocarbon such as isobutane)in the purge column. As shown, stage 1 (486) is the bottom of thecolumn, stage N (482) is the top of the column, the polymer being purgedis fed at the top (at 492) of the column, and the purge gas (e.g.,nitrogen) is fed at the bottom (at 488) of the column. The hydrocarbonpurge gas leaves the top (at 490) of the column and the polymer leavesthe bottom (at 494) of the column. The material balance for a component(i) for a stage (k) in the column is therefore,

F _(ik+1) +G _(ik−1) =F _(ik) +G _(ik) ;k=1, . . . ,N  (5).

The terms F_(ik+1) and F_(ik) are, respectively, the component massflows in the polymer entering and leaving the stage. Similarly, G_(ik−1)and G_(ik) are the component mass flows for the gas entering and leavingthe stage. The mass balance in equation 5 can be expressed in terms ofcomponent degassing rate for a stage (E_(ik)) as follows,

E _(ik) =F _(ik+1) −F _(ik) =W _(PE)(X _(ik+1) −X _(ik));k=1, . . .,N;i=1 . . . NC  (6a)

E _(ik) =G _(ik) −G _(ik−1) =G _(k) Y _(ik) −G _(k−1) Y _(ik−1) ;k=1, .. . ,N;i=1 . . . NC  (6b).

Equation 6a gives the material balance for the polymer-phase in thecolumn, while equation 6b gives the material balance for the gas-phasein the column. The X_(ik+1) and X_(ik) (equation 6a) are the massfractions of component in the polymer entering and leaving the stage,while Y_(ik−1) and Y_(ik) (equation 6b) are the corresponding massfractions for the purge gas for the stage. Equation 6a assumes that theamount of hydrocarbon dissolved in the polymer is small compared to thequantity of the polymer.

In these embodiments, the above material balance equations assume thatthe hydrocarbon contents of the polymer (F_(iN+1) and X_(iN+1)) fordegassing in the purge column are known. In the loop process, F_(iN+1)and X_(iN+1) can be estimated from the upstream flash gas (e.g., flashgas 226) composition of either the upstream flash vessel (e.g., flashvessel 224, high pressure flash, intermediate pressure flash vessel, lowpressure flash vessel, etc.), assuming equilibrium between the flash gasand the polymer. Of course, both flow and composition are usually knownfor the purge gas (i.e. G_(i0) and Y_(i0)). Thus, in this example, thematerial feeds to the column are defined. The model estimates thematerial streams (flow and composition) leaving the column: F_(i1) andX_(i1) for the polymer and G_(iN) and Y_(iN) for the gas.

In this example, assuming spherical particles for the polymer and usingcorrelations by Qi et. al. (1996), one can estimate the mass fraction ofcomponents for the polymer for a stage in the column as follows,

$\begin{matrix}{X_{ik} = {{\left( {X_{{ik} + 1} - X_{isk}} \right)\Phi_{ik}} + X_{isk}}} & (7) \\{\Phi_{ik} = {\sum\limits_{n = 1}^{\infty}{\frac{6}{n^{2}\pi^{2}}{\exp\left( {- \frac{n^{2}\pi^{2}D_{i}t_{k}}{R_{p}^{2}}} \right)}}}} & \left( {8a} \right) \\{t_{k} = {T_{res}/N}} & \left( {8b} \right)\end{matrix}$

where:

X_(isk)=Surface mass fraction of component for polymer particle

D_(i)=Diffusion coefficient for component for polymer (sqcm/s)

t_(k)=Residence time for polymer for stage k (s)

R_(p)=Mean radius for polymer particle (cm).

If the component in the gas leaving a stage is assumed to be inequilibrium with the component on the surface of the polymer leaving thestage, the surface concentration for component in the polymer can beestimated from Hutchinson and Ray correlation (1987), for example, asfollows,

X _(isk)=α_(v) *KVLE _(i) *P _(atm) *y _(ik) *MW _(i)/(1000*ρ_(PE))  (9)

Log(KVLE _(i))=−2.38+1.08(T _(ci) /T)²  (10a)

α_(v)=(ρ_(cr)−ρ_(PE))/(ρ_(cr)−ρ_(am))  (10b)

where:

α_(v)=Fraction amorphous phase for polymer

KVLE_(i)=Equilibrium constant for component for polyethylene

P_(atm)=Purge column pressure (atm)

y_(ik)=Mole fraction for component for purge gas leaving stage k

MW_(i)=Molecular weight of component

ρ_(PE)=Polymer density (g/cc)

T_(ci)=Critical temperature for a component

T=Temperature of purge column

ρ_(am)=Amorphous density of polymer (˜0.852 g/cc)

ρ_(cr)=Crystalline density of polymer (˜1.01 g/cc).

Hutchinson and Ray's correlations (equations 9-10) may accommodate awide range of polyethylene resins, from the amorphous (e.g. LLDPE) tothe less amorphous (e.g. HDPE). These exemplary correlations, therefore,are suitable for estimating hydrocarbon and non-hydrocarbon solubilityin all polyethylene resins, from the loop slurry process or otherprocesses.

D. Model Solution

In certain embodiments, the model may be solved by closing the overallmaterial balance for the column, expressed as,

DF _(i) =F _(iN+1) +G _(i0) −F _(i1) −G _(iN)  (11).

The term DF_(i) is the difference in material flow to and from thecolumn for a component. In the examples, the model solution meansdriving this difference (DF_(i)) to zero or to a negligible value (e.g.,10⁻⁶) for all components in the column. This solution method for thecolumn is generally iterative with material flows to and from the columncounter-current, with the feed streams (polymer and purge gas) known,and the product streams (degassed polymer and hydrocarbon-rich vapor)typically unknown a priori. Thus, in this example, an initial value ofone of the product streams from the column, vapor (G_(iN)) or polymer(F_(i1)), is estimated (e.g., an initial guess based on operatingexperience) to begin the column solution. The estimated stream is variediteratively until generally all material balances for the column areclosed. As an example, the vapor stream leaving the top of the columnmay be assumed and the steps in the algorithm in Table 2 followed tosolve the material balance for the column.

TABLE 2 Exemplary Algorithm for Solving Purge Column Model 1. Assumevapor flow from the column (i.e. G_(iN), y_(iN)). 2. Estimate fromy_(iN) and equation 9 the surface concentration (X_(isN)) of componentfor polymer particles leaving stage N. 3. Estimate from X_(isN) andequation 7 the bulk concentration of component (X_(iN)) for the polymerleaving stage N, the amount still adsorbed in the polymer (F_(iN) =W_(PE)X_(iN)), and the amount degassed for stage N (i.e. E_(iN) =W_(PE)(X_(iN+1) − X_(iN)). Note that the feed composition of polymer(X_(iN+1)) to the column is known. 4. Estimate from the material balancein equation 6(a, b) the vapor feed to stage N from stage N−1 (i.e.G_(iN−1) = G_(iN) − E_(iN)) and its mole fraction (y_(iN−1)). 5. Repeatssteps 2-4 for stages k = N−1, N−2, . . . , 2,1. 6. Evaluate DF_(i)(equation 11) to check for material balance closure for the column. a.If |DF_(i)| = 0 (or 10⁻⁶), model solved and stop iteration. b. If|DF_(i)| > 0 (or 10⁻⁶), continue iteration by adjusting the vapor flowfrom the column (G_(iN) and y_(iN)) and repeating steps 2 - 6 until step6a is satisfied.

E. Model Construction and Interface

As an example, the model may be implemented in Visual Basic Applicationswith an Excel worksheet as an interface (see, e.g., FIG. 19) for modelinput and output. Instructions for running the model may be provided tothe user or for automatically operating the model. FIG. 19 depicts anexemplary interface 500 for the purge column models. The interface 500is a spreadsheet having parts or boxes 502, 504, 506, 508, 510, 512,514, 516, 518, and 520 for inputting data (e.g., into spreadsheet cellswithin the boxes). These parts or boxes may also provide model output inspreadsheet cells. Input cells may be highlighted in a different colorthan output cells, for example. The inputs and outputs may be theindividual hydrocarbon component concentrations and the total VOC. VOCresults may be provided in box 522. Purge gas results may be input oroutput in box 424 in this exemplary interface As shown in the modelinterface of FIG. 19, apart from the primary purge gas feed 516 to thebottom of the column 228; the model can also simulate the impact onpurging the use of secondary purge gas 518 on a stage in the column 228.

In this exemplary interface, which may be displayed on a monitor of apersonal computer, for example, boxes 502 through 510 provide for modelinputs. Model results may also be displayed in these boxes andcalculated automatically by the model. Lastly, buttons 526, 528, and 530(e.g., with underlying Visual Basic commands) may be selected or clickedto operate the model based on different types of inputs and constraints.In this example, button 526 may be selected to estimate ppm compositionof the individual hydrocarbon components in the fluff feed to the purgecolumn (displayed in box 510) based on data supplied by the user inboxes 506 and 508.

In this exemplary interface, to run the model, the user may supply bedlevel data in box 504, and then select or click on (i.e., with a mousepointer) button 528 to run the command “Purge Fluff for Given Bed Level”to estimate the residence time of the purge column and the fluff VOCafter purging for that length of time. On the other hand, the user maysupply the purge time in box 504 and then select the button 530 to runthe command “Purge Fluff for Given Purge Time” to estimate the fluff VOCafter purging for that length time.

The predicted fluff VOC may differ from plant data, if known. In thiscase, known plant data may be placed in box 512 in this interface, andthe button 532 selected to run the command “Fit Purge Model to PlantData” to adjust diffusion parameters in the model to fit modelpredictions to the plant data. Given desired VOC for polymer, model canestimate the purge gas flow to achieve this goal. To do this, VOC targetdata may be entered in the interface in box 524, and the button 534selected to run the command “Estimate Purge Gas Given VOC” to estimatethe purge gas rate composition to satisfy the VOC target for the polymerfluff exiting the purge column 228.

F. Model Validation

Plant data were collected from polyethylene operating facilities forvalidating the model. Paired sample data was collected, a sample of thefeed from the flash tank to the purge column, and a sample of thepolymer fluff leaving the purge column. Sampling procedures for testingVOC in the manufacturing facility may involved connecting a samplecylinder to a sample port at the top or bottom of the purge column andopening and closing valves on the port and cylinder to collect thesample. The date and time of each sample collection may be recorded forthe retrieval of process data for the column from the plant datahistorian at later dates. The samples may be then analyzed in thelaboratory in the plant facility to measure the hydrocarbon content ofeach polymer fluff sample, within 10 minutes of collection, for example.Organic or hydrocarbon entrainment may be estimated for each sample. Anexemplary entrainment calculation includes estimating the void volumebetween polymer particles in the feed to the purge column and the massof flash gas from the flash tank that will be contained in this volume,as follows,

Ent_(i) =y _(i) *P*MW _(i) *W _(PE)(1/ρ_(b)−1/ρ_(PE))/RT  (12)

where:

Ent_(i)=Entrainment for a component (lb/hr)

MW_(i)=Molecular weight of a component

P=Flash tank pressure (atm)

R=Gas constant (0.7304 atm·cuft/lb-mole·R)

T=Flash tank temperature (F)

W_(PE)=Reactor production rate (lb/hr)

y_(i)=Flash gas component mole fraction

ρ_(b)=Fluff bulk density (lb/cuft)

ρ_(PE)=Polymer solid density (lb/cuft).

Entrainment corrections may or may not include hydrocarbons from processleaks, which are difficult to measure or even estimate. A strategy foraccounting for process leaks may be to add a fraction of gas from theflash tank to the entrainment gas, depending on the pressure differencebetween the flash tank and the purge column, and so on. In certainexamples, it is believed that such leaks could account for about 4% ofmaterial flows around the purge column in plants.

A summary of equation variables is listed below.

Notations

D_(bed) Purge column diameter (ft)D_(i) Coefficient of diffusion for a component (m²/s)DF_(i) Material balance error for a component (lb/hr)E_(ik) (Degassing rate of component for polymer (lb/hr)Ent_(i) Entrainment for a component (lb/hr)f_(bed) Bed level fraction in purge column (%)F_(ik) Flow rate for a stage for adsorbed component for polymer (lb/h)G_(k) Flow rate for a stage for purge gas (lb/h)G_(ik) Flow rate for a stage for a component for purge gas (lb/h)HETP Height of a stage in purge column (in)H_(bed) Inventory level of polymer in purge column (ft)H_(LL) Minimum inventory level for purge column (ft)H_(HL) Maximum inventory level for purge column (ft)MW_(i) Molecular weight of a componentKVLE_(i) Equilibrium constant for a component for polymerN Number of stages for purge columnNC Number of material components in the purge columnP Flash tank pressure (atm)P_(atm) Purge column pressure (atm)PPM Mass fraction of component in polymerR Ideal gas constant (0.7304 atm·cuft/lbmol·R)R_(p) Radius of polymer particle (m)T_(ci) Temperature of emulsion phase in the bed (F)T Purge column or flash tank temperature (F)t_(k) Purge column residence of polymer for a stage (s)T_(res) Purge column polymer residence time (h)W_(inv) Purge column inventory (lb)W_(PE) Polymer withdrawal rate for reactor (lb/h)X_(ik) Average mass fraction of component for polymerX_(isk) Mass fraction of component at polymer particle surfacey_(ik) Mole fraction of component for purge gas for a stage

Greek Letters

α_(v) Fraction of amorphous phase for polymerρ_(am) Density of amorphous polymer (g/cc)ρ_(cr) Density of crystalline polymer (g/cc)ρ_(b) Bulk density of polymer (lb/cuft)ρ_(PE) Density of polymer (g/cc)Φ_(ik) Parameter for a stage in equation 8a

Indices

i A component in the purge column or flash tankk A stage in the purge column.

1. A polyolefin production system comprising: a purge column configuredto purge a recovered polyolefin stream comprising polyolefin particlesand residual hydrocarbon with a purge gas to generate a discharge streamcomprising a first portion of the residual hydrocarbon and the purge gasand a polyolefin product stream comprising the polyolefin particles anda second portion of the residual hydrocarbon; and a computer configuredto calculate an estimated value representing a volatile organic content(VOC) of the polyolefin product stream based on a flow rate of thepolyolefin particles flowing through the purge column and a flow rate ofthe purge gas flowing through the purge column.
 2. The polyolefinproduction system of claim 1, wherein the purge column comprises acountercurrent purge column.
 3. The polyolefin production system ofclaim 1, wherein the computer is configured to estimate the valuerepresenting the VOC by iteratively driving a difference in flow valuesfor material flow to and from the purge column to zero for allcomponents of the purge column.
 4. The polyolefin production system ofclaim 1, comprising: a polymerization reactor configured to polymerizeolefin monomer in a hydrocarbon diluent in the presence of a catalyst toproduce a product slurry comprising the polyolefin particles andhydrocarbon; and a flash vessel configured to separate flashedhydrocarbon from the product slurry to generate a flash gas stream andthe recovered polyolefin stream.
 5. The polyolefin production system ofclaim 4, wherein the polymerization reactor comprises a continuoustake-off configured to discharge the product slurry from thepolymerization reactor.
 6. The polyolefin production system of claim 1,comprising a controller configured to adjust a residence time of thepolyolefin particles in the purge column based on the estimated valuerepresenting the VOC.
 7. The polyolefin production system of claim 1,comprising a controller configured to adjust a flow rate of the purgegas entering the purge column based on the estimated value representingthe VOC.
 8. The polyolefin production system of claim 1, comprising acontroller configured to adjust a temperature setting for the polyolefinparticles in the purge column based on the estimated value representingthe VOC.
 9. The polyolefin production system of claim 1, comprisingprocessing components configured to process the polyolefin particles toform a polyolefin product.
 10. A polyolefin production system,comprising: a purge column configured to receive a purge gas andconfigured to purge a recovered polyolefin stream comprising polyolefinparticles and residual hydrocarbon with the purge gas; a first flow pathfrom the purge column configured to pass a discharge stream generated bythe purge column, the discharge stream comprising a first portion of theresidual hydrocarbon and the purge gas; a second flow path from thepurge column configured to pass a polyolefin product stream comprisingthe polyolefin particles and a second portion of the residualhydrocarbon, wherein the second portion comprises a volatile organiccontent (VOC) of the polyolefin product stream; and a computerconfigured to calculate an estimated value representing the VOC of thepolyolefin product stream based on a flow rate of the polyolefinparticles flowing through the purge column and a flow rate of the purgegas flowing through the purge column.
 11. The polyolefin productionsystem of claim 10, comprising a control feature configured to adjust anoperating condition of the purge column based on the estimated valuerepresenting the VOC.
 12. The polyolefin production system of claim 10,wherein the purge column comprises a countercurrent purge column. 13.The polyolefin production system of claim 10, wherein the computer isconfigured to estimate the value representing the VOC by iterativelydriving a difference in flow values for material flow to and from thepurge column to zero for all components of the purge column.
 14. Thepolyolefin production system of claim 10, comprising a polymerizationreactor configured to polymerize olefin monomer in a hydrocarbon diluentin the presence of a catalyst to produce a product slurry comprising thepolyolefin particles and hydrocarbon.
 15. The polyolefin productionsystem of claim 14, comprising a flash vessel configured to separateflashed hydrocarbon from the product slurry to generate a flash gasstream and the recovered polyolefin stream.
 16. The polyolefinproduction system of claim 15, wherein the polymerization reactorcomprises a continuous take-off configured to discharge the productslurry from the polymerization reactor.
 17. The polyolefin productionsystem of claim 15, wherein the polymerization reactor comprises asettling leg configured to discharge the product slurry from thepolymerization reactor.
 18. A polyolefin production system comprising: apolymerization reactor configured to polymerize olefin monomer in ahydrocarbon diluent in the presence of a catalyst to produce a productslurry comprising polyolefin particles and hydrocarbon; a flash vesselconfigured to separate flashed hydrocarbon from the product slurry togenerate a flash gas stream and a recovered polyolefin stream. a purgecolumn configured to purge the recovered polyolefin stream with a purgegas, the recovered polyolefin stream comprising the polyolefin particlesand residual hydrocarbon; a first flow path configured to pass adischarge stream generated by the purge column, the discharge streamcomprising a first portion of the residual hydrocarbon and the purgegas; a second flow path configured to pass a polyolefin product streamgenerated by the purge column comprising the polyolefin particles and asecond portion of the residual hydrocarbon; and a computer configured tocalculate an estimated value representing a volatile organic content(VOC) of the polyolefin product stream based on a flow rate of thepolyolefin particles flowing through the purge column and a flow rate ofthe purge gas flowing through the purge column.
 19. The polyolefinproduction system of claim 18, wherein the purge column comprises acountercurrent purge column.
 20. The polyolefin production system ofclaim 1, wherein the computer is configured to estimate the valuerepresenting the VOC by iteratively driving a difference in flow valuesfor material flow to and from the purge column to zero for allcomponents of the purge column.